[flang][driver] Introduce FCC_OVERRIDE_OPTIONS. (#140556)
This PR add functionality to change `flang` command line using
environment variable `FCC_OVERRIDE_OPTIONS`. It is quite similar to what
`CCC_OVERRIDE_OPTIONS` does for clang.
The `FCC_OVERRIDE_OPTIONS` seemed like the most obvious name to me but I
am open to other ideas. The `applyOverrideOptions` now takes an extra
argument that is the name of the environment variable. Previously
`CCC_OVERRIDE_OPTIONS` was hardcoded.
